Nanoholes improve nanowire-based solar cells

Photovoltaic devices need to be more cost-effective in order to make these renewable energy sources competitive with traditional fossil fuels. One option is to use hybrid solar cells that combine silicon nanowires with low-cost, photoresponsive polymers. The high surface area and confined nature of nanowires allows them to trap significant amounts of light for solar cell operations. However, these thin, needle-like structures are very fragile and tend to stick together when the wires become too long.

Now, findings by Xincai Wang from the A*STAR Singapore Institute of Manufacturing Technology and co-workers from Nanyang Technological University could turn the tables on silicon nanowires by improving the manufacturing of silicon nanoholes, which are narrow cavities carved into silicon wafers that have enhanced mechanical and light-harvesting capabilities.

A straightforward procedure that transforms silver nanospheres (top) into silicon nanoholes (bottom) can overcome the shortcomings of nanowire-based solar cells.

Nanoholes are particularly effective at capturing light because photons can ricochet many times inside these openings until absorption occurs. Yet a practical understanding of how to fabricate these tiny structures is still lacking. One significant problem, noted Wang, is control of the initial stages of nanohole formation, a crucial period that can often induce defects into the solar cell.

Instead of traditional time-consuming lithography, the researchers identified a rapid, maskless approach to producing nanoholes using silver nanoparticles. First, they deposited a nanometre-thin layer of silver onto a silicon wafer that they toughened by annealing it using a rapid-burst ultraviolet laser. Careful optimisation of this procedure yielded regular arrays of silver nanospheres on top of the silicon surface, with sphere size and distribution controlled by the laser annealing conditions.

Next, the nanosphere-silicon complex was immersed into a solution of hydrogen peroxide and hydrofluoric acid, a mixture that eats away at silicon atoms directly underneath the catalytic silver nanospheres. Subsequent removal of the silver particles with acid produced the final, nanohole-infused silicon surface.

The team analysed the solar cell activity of their nanohole interfaces by coating them with a semiconducting polymer and metal electrodes. Their experiments revealed a remarkable dependence on nanohole depth: cavities deeper than one micrometre showed sharp drops in power conversion efficiency from a maximum of 8.3 per cent due to light scattering off of rougher surfaces and higher series resistance effects.